1. Field of the Invention
The present invention relates to an ophthalmologic apparatus capable of taking photofluorography.
2. Description of the Related Art
A known photography apparatus, for observing an examinee eye, picks up an image of an examinee eye with an image pickup device, as represented by a CCD, and converts it into an image signal. Since an optical finder cannot be used, especially when taking infrared photofluorography with a fundus camera, alignment and focusing are carried out using an image pickup device.
In general, a radiographic contrasting period of the photofluorography is classified into three stages: a radiographic contrast early phase (1) where the stage is from the starting of contrasting a choroid coat to contrasting a choroidal vein; a radiographic contrast medium phase (2) where the stage is until a fluorescence agent is vanished in the choroidal vein; and a radiographic contrast late phase (3) where the stage is defined as when the diffuse background fluorescence of the choroidal vein is viewed. A fluorescence agent injected in a vein of an examinee is circulated together with blood so as to first arrive into a thick blood vessel in an eye-ground in the radiographic contrast early phase, and then, the agent gradually penetrates thin blood vessels with time through the medium and the late phase.
Hence, the concentration of the fluorescence agent existing in the blood vessel is higher in the radiographic contrast early phase than that in the radiographic contrast late phase. Accordingly, during taking infrared photofluorography, the examinee eye in the early phase is very bright due to the circulation state of the fluorescence agent in comparison with that in the medium and late phases, and the change in its brightness is also large. Thus, the dynamic range of the fluorescence luminance is largely increased in comparison with that of the image pickup device, so that it is very difficult to uniquely determine the photography light amount and the amplification factor of the image pickup device for obtaining excellent image quality in both the low fluorescence luminance and the high fluorescence luminance.
In order to solve this problem, a technique (a) using an auto gain control (a so-called AGC) was proposed in that while observing the examinee eye, the fluorescence luminance is changed and a constant image signal can be stably obtained. In this technique, during fluorescence observation, the average output of the image signal from the examinee eye is controlled to be stably constant, where even the brightness of the examinee eye and the observing light amount are changed by autonomously changing the amplification factor of the image pickup device. On the other hand, during picking up still images, the luminous period of time of a photography light source is short, several micro seconds, so that even if the AGC is operated, the shooting cannot be tracked. Then, the amplification factor control is changed from the AGC system to a fixed gain system, and the photography light amount is adjusted so as to have an appropriate exposure from the amplification factor of the image pickup device, the observing light amount, and the brightness of the examinee eye before the shooting for stably optimizing the picked up images of the examinee eye.
Also, in order to simply adjust the photography light amount to have appropriate exposure even as the fluorescence luminance in an eye-ground decreases with time after starting the photofluorography, a device and a photography technique (b), both having a timer for detecting an elapsed time after the injection into a vein, are discussed in Japanese Patent Laid-Open No. 2-124137, which describes a photography technique where the photography light emission amount is increased in accordance with the elapsed time after the injection into a vein when the elapsed time signal is received from the timer.
In the related art describing technique (a), during the photofluorography, three operations must be instantly performed, which are: the reading the amplification factor of the AGC during observation direct before the shooting; the calculation of the optimal amplification factor and light emission amount; and the setting of the exposure value. Thus, the workload to be carried out by the ophthalmologic apparatus is large in a period between the starting and completion of the shooting, resulting in a more complicated system.
In the related art describing technique (b), the relationship between the photography elapsed time and the fluorescence luminance in an eye-ground is largely different due to personal factors in equations of an examinee, such as age, sexuality, body weight, body height, and any disease, or the photographic difference due to an examiner, such as the amount of the fluorescence agent to be injected into a vein and the injection rate into the vein. Therefore, in a technique of increasing the photography light amount with only the parameter of the elapsed time after the injection into the vein like this technique, the halation in the radiographic contrast early phase and the contrast shortage in the late phase cannot be avoided, making the shooting difficult.